…2026.3.22 sync drift (#2508)

Upstream sync `0667aa5596` (v2026.3.22, #2400) replaced the config
sidebar + nav + subnav + tag-picker CSS family with a flat
`.config-top-tabs` horizontal bar as part of a dashboard-v2 redesign.
The fork's `ui/src/ui/views/config.ts` kept the vertical sidebar
layout (with icons, tag picker, and two-level subsection nav) —
features upstream removed entirely. Result: settings view shipped
with 21 orphaned class references (reported by
`scripts/audit-css-class-drift.mjs` cluster 1).

Third confirmed instance of the "definition-site sync without paired
call-site update" variant documented in HQ #57 (after #2493 and
#2501).

## Approach

Fork-owned CSS restore (hybrid pattern from #2506): add a fork-owned
`ui/src/styles/config-sidebar.css` with pre-sync rules, imported
AFTER upstream `config.css` so the `.config-layout` grid override
cascades. Zero TS changes — config.ts kept as-is.

Full TS rename to upstream vocabulary (sidebar → config-top-tabs)
was rejected: upstream's layout is horizontal flat tabs with no
icons / no tag picker / no subnav. A rename without restructuring
would inherit upstream's horizontal layout and break the sidebar
UX. Full restructure is a product change, not a drift fix.

## Changes

- `ui/src/styles/config-sidebar.css` (new, 372 LOC): restored rules
  for .config-sidebar/__header/__title/__footer, .config-nav family,
  .config-subnav family, .config-search__hint/__hint-label, and the
  .config-search__tag-* picker family. Includes .config-layout
  two-column grid override and mobile breakpoints (768px, 480px).
  Theme selector migrated from pre-sync `[data-theme="light"]` to
  current `[data-theme-mode="light"]` convention (matches 30×
  occurrences in config.css, 23× in components.css). Hardcoded
  legacy brand color `rgba(255, 77, 77, 0.4)` in
  `.config-subnav__item.active` replaced with
  `color-mix(in srgb, var(--accent) 40%, transparent)` for theme
  consistency (matches established idiom, 28× across styles tree).
- `ui/src/styles.css` (+1 LOC): import the new file immediately
  after upstream `config.css` so cascade overrides work.

Fork-owned CSS is conflict-free against future upstream syncs of
`config.css` — upstream has zero rules for the 21 restored classes.

## Scope notes

- AC3 (`pnpm dev` visual smoke of sidebar/tabs, tag picker, subnav)
  requires human browser pass — same as #2506's unchecked item.
  Deterministic parts fully verified: exact-match sweep confirms
  21 emitted classes ↔ 21 defined classes.
- Peer tracks in this wave: #2509 (cluster 2 — `04a7853f0f`
  theme-toggle/nav) and #2510 (cluster 3 — `21ac4b9a8a` `.btn-sm`).
  Different files, no conflict.
- Not LIVE-smoke-test triggering: touches `ui/` only, no
  `src/middleware/` changes.

## Test plan

- [x] `pnpm check` (format:check + tsgo + lint + lint:tmp + lint:no-remoteclaw-ai) — all pass
- [x] `pnpm canvas:a2ui:bundle` — builds (457 kB, 66 ms)
- [x] `pnpm vitest run --config vitest.unit.config.ts ui/src/ui/` — 4 files / 43 tests pass
- [x] Fork-integrity gates: `check-rebrand-leakage.sh`, `check-no-zombie-imports.mjs`, `check-stub-debt.mjs`, `check-throwing-stub-callers.mjs` — all pass
- [x] `node scripts/audit-css-class-drift.mjs` — cluster 1 (0667aa5) 21 → 0 orphans
- [x] Exact-match sweep — 21 classes emitted by config.ts ↔ 21 classes defined in config-sidebar.css (no missing, no extra, no typos)
- [ ] Visual smoke — load settings view in light + dark themes, verify sidebar, tag picker, subnav render correctly (requires human browser pass)

## References

- Closes #2508
- Audit parent: #2502
- Sync commit: `0667aa5596` (#2400)
- Similar fix precedent: #2501 / #2506 (v2026.3.13-1 nav + topbar drift)
- HQ post-mortem: remoteclaw/hq#57 (now 3 confirmed instances)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>